|This is the talk page for discussing improvements to the Fresnel zone article.
This is not a forum for general discussion of the article's subject.
|This article is of interest to the following WikiProjects:|
- 1 Untitled
- 2 General formula is fine, but numerical values only for optics in free-space
- 3 zone/region ?
- 4 Error in Attached Image
- 5 Article is written to refer only [or predominantly] to radio waves
- 6 Pronunciation discontinuity
- 7 Question about phase information
- 8 Confusions
- 9 One of the Worst Technical Articles on Fresnel Zones
- 10 resummarized
- 11 Assessment comment
- 12 Shape around the antenna
needs more reliable sources....--184.108.40.206 02:10, 2 November 2006 (UTC)
what's the unit of r ? what about the fomula in metric ?
- I guessed it is "kilobarleycorns". Probably wrong, I suppose. What are the widths of the subsequent Fresnel zones, those "annular rings"? What are the widths at other than the halfway point?
- What do "maximum obstruction" and "recommended obstruction" mean? Gene Nygaard 04:29, 4 May 2005 (UTC)
This article needs to be generalized to include the optics usage. (Yes, I know, sofixit and all that. I don't have time right now, so I'll just mention it here for now.)--Srleffler 04:07, 21 March 2006 (UTC)
The link to "this page" above appears to be no longer reachable, but I've seen 43.3 elsewhere and had the same question. See this page instead. The 43.3 multiplier (for the radius in feet and distance in miles) may be the "obstacle-free radius", which is often taken as 60% of the Fresnel zone radius. Can someone verify this? 220.127.116.11 21:50, 25 October 2006 (UTC) Gerald Reynolds
General formula is fine, but numerical values only for optics in free-space
I haven't even checked the accuracy of those numerical prefactors, but people should be aware that the concept of a Fresnel zone applies as well to *any* wave phenomenon (my own familiarity with it is in acoustics). I'll change the article if and when I have time, but (1)it should be mentioned that those numerical values which people have been quibbling over are specific to optics in free-space---other values will show up for acoustics problems---still others for other wave-types in different media; or, (2) there should be *no* numerical values given, and simply stress that the formula yields different results based on wave- and media-type. --Smoo222 03:20, 21 March 2007 (UTC)smoo222
Should this page make any distinction between "Fresnel zone" and "Fresnel region" (commonly used in antenna theory)?
In antenna theory, the Fresnel region generally refers to a radial range of distances between the reactive near field (~2*lambda?) of an aperture, and the Fraunhofer region (2*D^2/lambda, where D is the largest dimension of the aperture). Within the Fresnel region, the radiation pattern of the aperture varies significantly with radial distance since the multitude of sources that constitute a given aperture cannot yet accurately be approximated as having a common phase center.
18.104.22.168 23:42, 21 March 2007 (UTC)Bill Shultz
This is written like a high-school science report. Please recast it in, at the very least, the passive voice.
Error in Attached Image
Currently there's a spurious '15m' above the '20m' on the bottom set of images. It seems to have been left in there by mistake. Xrobau 12:50, 10 September 2007 (UTC)
Article is written to refer only [or predominantly] to radio waves
The article, while apparently sufficient for radio transmitter/receiver problems, mentions almost nothing of other wave types, which of course the entire concept applies to. A specific example is that of mentioning "radio frequency line-of-sight": There is absolutely NO restriction to radio frequencies.
There is also a minor correction which should be made (I have no time to get it totally right), in that it is mentioned that "If unobstructed, radio waves will travel in a straight line from the transmitter to the receiver." This is true at some level of abstraction (that of infinitely high frequency -- the domain of "ray optics"), but *false in the domain where Fresnel zone concepts apply*. In other words, the Fresnel zone is exactly one of several concepts designed to deal with the fact that waves do not travel in straight lines, even when there are no obstructions!
Another suggestion (again, I'll fix it if/when I have time) is to mention the source/receiver reciprocity which is implied by the figures included in the article. For an arbitrary receiver location (imagine the _field_ from the source), the Fresnel zone does not pinch down as is shown in the figures. The "pinching" is a result of the _receiver's_ own Fresnel zone -- that is, its response to an oriented incoming plane wave because the receiver also has a finite width aperture, and an orientation. The cigar-shaped zone in the figures is the *product* of the individual Fresnel zones of the source and receiver, taken individually. Smoo222 (talk) 13:30, 31 March 2009 (UTC)Smoo222
Why is Fresnel pronounced "/frɛnɛl/ fre-NELL" in this article, but "(pronounced /freɪˈnɛl/ fray-NELL)" in the Fresnel lens article? Both refer to the same person. I'd fix it, except that I've always pronounced it /frɛz'nɛl/ (frez-nell). My attempt is probably incorrect, but it makes me unaware of which is correct.
It seems as if the pronounciation has been "fixed" in the Fresnel lens fashion while the correct (French!) pronounciation should probably be "/frɛnɛl/ fre-NELL"!22.214.171.124 (talk) 15:48, 4 February 2015 (UTC)
Question about phase information
I don't understand the assertion that an obstacle in the first Fresnel zone will result in an interfering signal 0-90 degrees out of phase, 90-270 degrees if in the second zone, etc.
In the first place, surely the phase change associated with a path that goes from one antenna to point P on the surface of the first Fresnel zone to the second antenna is 180 degrees (one half-wavelength path difference).
In the second place, there will likely be a phase change associated with the object causing the interference. For a specular reflection (e.g., a plane surface many wavelengths in extent, such as a flat roof), the phase change will depend on the angle of incidence, polarization and nature of the surface (e.g., dielectric, highly conducting, etc.) and can vary easily from 0-180 degrees.
You are correct, this can be very confusing given that the information is erroneous. I saw a webpage that could be the source for this wrong information. I'll fix it to say 0-180 180-360 and so on and add that this is the path-length phase difference. Maxbezada (talk) 20:47, 30 January 2013 (UTC)
Some of this might already be found in the article or in other comments but I think the article needs substantial clarifications to avoid confusions and misunderstandings.
"Fresnel zones result from diffraction by the circular aperture". No, diffraction by the circular aperture will cause sidelobes in the radiation pattern of circular antennas, but the Fresnel zones are independent of the antennas used (even though both could be important for the end result).
As I understand it the Fresnel zones describes areas of different path differences between the direct beam and an indirect beam that is scattered (e.g. reflected) from some position outside the line of sight. The article may actually say this but not in a very clear way. Since interference depends on phase rather than path differences the path difference is recalculated into a phase difference or difference in wavelengths, lambda. It needs to be clarified whether the zones refers to volumes (the introductory part describes the cross section of the first as circular and the subsequent as annular, which would require the zones to be volumes and a zone in 3D space would normally be interpreted as volume) or if they refer to the limiting surfaces, the distinction does not seem to be well made.
If it is the limiting surface Fn is the zone radius, but if it is a volume it is the outer radius of this volume.
If it is a surface the path difference will be an integer number of wavelengths for the even zones but 0.5*n*lambda for the odd zones. Below I will sometimes refer to the limiting surface as the surface to distinguish it from some zone volume.
If it is a volume the phase difference due to path difference will vary from (n-1)pi to n*pi in the n:th zone, which means that it could either be in-phase or out of phase or have some intermediate phase relation with the direct signal.
In addition to the path difference the scattering process can give rise to phase differences. Often reflexions will lead to a 180 degree phase shift making rays reflected from the n:th surface out of phase with the direct beam. Thus reflexions from the first surface could enhance rather than cancel the signal. Above the Brewster angle reflexions (for beams of vertical polarisation) will not change phase. Scattering may also lead to intermediate phase shifts (typically for lossy media near the Brewster angle) and the end result may also depend on phase shifts of the antenna outside the boresight direction (but since the Fresnel zones are often reasonably narrow compared to the main lobe of the antenna this may rarely be important).
The formula for the Fresnel zone radius appears to be an approximation. Although it is probably a very good approximation in most cases it is good to indicate this with a "curly equal sign" and to clearly state that the radius corresponds to a path difference of 0.5*n*lambda.
One of the Worst Technical Articles on Fresnel Zones
This article is wrong on many levels, including its reference to circular apertures and diffraction (concepts also associated with Fresnel, but having no relationship to Fresnel zones). Specifically, the stated formula is trivially derived based on the difference between the direct path length and alternate path lengths being a multiple of a half-wavelength between two points (i.e., the transmitting and receiving antennas):
Fn = The nth Fresnel Zone radius in metres
d1 = The distance of P from one end in metres
d2 = The distance of P from the other end in metres
= The wavelength of the transmitted signal in metres
The various Fresnel zone ellipsoids do not "define volumes in the radiation pattern of a (usually) circular aperture": they define the respective terminating surfaces for alternate path lengths that differ by the various multiples of a half-wavelength.
- I would put it this way: the article tells you how to calculate an ellipsoid. What this has to do with wave transmission, reflection from obstacles, interference patterns, or anything like that, you'll have to learn elsewhere, because the author of this article is keeping pretty quiet on those points. But you can visualize the ellipsoid. 126.96.36.199 (talk) 15:50, 10 June 2015 (UTC)
That was about the worst summary I've ever seen on an article. I know what fresnel zones are, and even I was confused after reading it. Bad phrasing, extraneous material, parenthetical extras, weasel words, ick. I just rewrote it from scratch and then tried to see if there was anything I left out that was in the original text that could be salvaged. I leave it up to someone else to make it more accurate, but I think now it is at least understandable. --ssd (talk) 04:50, 24 August 2015 (UTC)
The comment(s) below were originally left at several discussions in past years, these subpages are now deprecated. The comments may be irrelevant or outdated; if so, please feel free to remove this section., and are posted here for posterity. Following
|It is not necessary to explain this in such technical terms.|
Last edited at 23:53, 28 January 2007 (UTC). Substituted at 15:36, 29 April 2016 (UTC)
Shape around the antenna
The antennas are in the focal points of the Fresnel zone, not at its boundary, as stated in "The cross sectional radius of each Fresnel zone is ..., shrinking to a point at the antenna on each end." Petr Matas 14:37, 4 June 2016 (UTC)